German Patent Application No. DE 199 03 439 describes a method and a device for operating an internal combustion engine in whose exhaust region an SCR (selective catalytic reduction) catalytic converter is disposed, which reduces the nitrogen oxide contained in the exhaust gas of the internal combustion engine to nitrogen with the aid of a reagent. The reagent is preferably dosed as a function of operating parameters of the internal combustion engine such as the rotational speed and the injected fuel quantity. Moreover, the dosing is preferably implemented as a function of exhaust-gas characteristics such as the exhaust-gas temperature or the operating temperature of the SCR catalytic converter.
The reducing agent ammonia, for instance, which may be obtained from a urea-water solution, is provided as reagent. The dosing of the reagent or of starting materials of the reagent must be specified with care. If the dosage is too low, the nitrogen oxide in the SCR catalytic converter will no longer be reducible completely. If the dosage is too high, reagent slippage occurs, which not only results in an unnecessarily high reagent use but also in an unpleasant smell as a function of the reagent's composition.
In German Patent Application Nos. DE 199 60 731 and DE 199 62 912, NOx sensors are described, which detect the NOx concentration present in an exhaust-gas stream. The described NOx sensors have a plurality of chambers, which are interconnected via the diffusion barriers. Due to the measuring principle the known multi-chamber NOx sensors have a cross sensitivity with respect to ammonia (NH3). The ammonia—as an example of a reagent—contained in the exhaust gas leads to a falsification of the sensor signal via the reactions 4NH3+5O2→4NO+6H2O.
Therefore, if the reagent dosage is increased in the previously known methods and if overdosing or correct dosing of the reagent takes place, the sensor signal will increase because of the occurring reagent slippage, and if underdosing has occurred, the sensor signal will decrease due to the increasing NOx conversion. However, if the reagent dosage is reduced and overdosing of the reagent is present, the sensor signal will decrease because of the reduced reagent slippage, and if correct dosing or underdosing is present, it will increase because of the no longer complete NOx conversion.
In German Patent Application No. DE 10 2004 046 640 (not prepublished), a method for operating an internal combustion engine and a device for implementing the method are described in which an NOx sensor having cross sensitivity with respect to a reagent is situated downstream from an SCR catalytic converter. At least one SCR catalytic converter, which is acted upon by the reagent contributing to the NOx conversion in the SCR catalytic converter, is disposed in the exhaust-gas region. The calculation of at least one measure of the NOx concentration occurring downstream from the SCR catalytic converter is provided, which allows the dosage of the reagent to be specified with greater precision. A reagent slippage may be calculated from the difference between the calculated measure of the NOx concentration and the measured measure of the sum of the NOx concentration and the reagent concentration. Taken into account is the fact that both a reagent slippage and also an insufficient NOx reduction reaction produce a deviation in the same direction between the calculated measure of the NOx concentration and the measured measure of the sum of the NOx concentration and the reagent concentration. If the difference is too high, the dosage of the reagent is initially reduced according to one development. If reagent slippage has occurred, the lowering of the reagent dosage will result in reduced reagent slippage. In this case the reduced dosing of the reagent has turned out to be the correct measure. If the dosage of the reagent was too low originally, the ascertained difference will increase further due to the lower NOx conversion, so that it can be inferred from this that the reduction in the dosing of the reagent was wrong and the dosage should be increased instead.
In German Patent Application No. DE 10 2004 031 624 (not prepublished), a generic method for operating an internal combustion engine and a device for implementing the method are described in which an NOx sensor having cross sensitivity with respect to a reagent is once again situated downstream from the SCR catalytic converter. An open-loop control or closed-loop control of the reagent level in the SCR catalytic converter to a predefined reagent setpoint level is provided. The controlled specification of the reagent setpoint level not only ensures that a sufficient reagent quantity for the optimally possible complete removal of the NOx raw emissions of the internal combustion engine is available in non-stationary states of the internal combustion engine, but also that no reagent slippage occurs.
The reagent level in the SCR catalytic converter is ascertained with the aid of a catalytic converter model, which takes into account the NOx mass flow entering the SCR catalytic converter, the NOx mass flow leaving the SCR catalytic converter, the catalytic converter temperature and possibly the reagent slippage. The reagent level of the SCR catalytic converter depends on the operating temperature of the SCR catalytic converter, in particular, which is highest at low operating temperatures and drops to lower values as the operating temperature rises. The efficiency of the SCR catalytic converter depends on the catalytic activity, which is low at low operating temperatures, runs through a maximum with an increasing operating temperature, and then drops again as the operating temperature increases further. According to a preferred development, the predefined difference between the maximally possible reagent level and the reagent setpoint level should not fall below a specified measure such as 20%, so that in a sudden, pronounced temperature increase the desorbed reagent quantity in the SCR catalytic converter is able to react with the offered NOx quantity, so that reagent slippage is avoided in all operating situations.
The present invention is based on the objective of providing a method for operating an internal combustion engine in whose exhaust-gas region an SCR catalytic converter is situated, and a device for implementing the method, which ensure the greatest possible NOx conversion in all operating situations, if possible.